Update .travis.yml: fix build failures and add 7.6.3

Summary:
 * Always install ghc from hvr's repo (simpler and not a big cost)
 * Install recent alex and happy (this fixes build failures with 7.8)
 * Debug: print versions of ghc, cabal, alex, happy
 * Include benchmarks
 * Cleanup:
   * cabal instead of $CABAL
   * remove lens specific stuff
   * reorganize what goes into before_install, install and script